Office equipment, such as printers and copiers, which place images based on digital data onto sheets, such as sheets of paper, are well known. More sophisticated types of office equipment are capable of placing images on both sides of a single sheet of paper, a feature often referred to as “duplexing.” A typical configuration of a duplexing printer (the word “printer” including other types of equipment, such as digital copiers and facsimile machines) will include a marking device, meaning some hardware which places a desired image on a sheet, which is capable of printing only on one side of the sheet at a time. In order to print on both sides of the same sheet, it is necessary to feed a sheet through the marking device the first time, so the sheet can receive a first image on one side thereof, and then invert the sheet and re-feed it back into the marking device so that the marking device can place a second image on the other side of the sheet. Although the specific architectures of various office equipment on the market varies widely, the path (along with any associated sheet-handling hardware, such as belts or rollers and motors) by which a sheet has been output by the marking device is inverted and re-fed to the marking device can be generally referred to as a “duplex path.” In the market for office equipment having duplex features, a common customer requirement is a precise registration between an image printed on one side of the sheet with the image printed on the other side. However, positioning an image on one side of a sheet in a manner that coincides with the position of the image on the other side of the sheet can be difficult.
Registration of a first image on a first side of a sheet with a second image on a second side of a sheet is not always accurate because of one or more registration errors that offset the first image relative to the second image. For example, a page number printed on the bottom-center position of the first side of a two-sided, printed document should align exactly with the page number printed on the reverse side. The offset of the page number on the second side of a sheet with respect to the page number on the first side of the sheet is a registration error that is extremely undesirable, and considered unacceptable in various printing industries.
Registering two images on the front and back sides of a single sheet of media sheet is important in industries such as the offset printing industry. In this industry, duplex sheets are sometimes produced having a number of pages of what will ultimately be a single, multi-page document, aligned on the front and back of a single sheet of media sheet. To create such a multi-page document, a sheet of media sheet is printed with multiple images on the front and back side of a single composite sheet. The single composite sheet is subsequently folded and segmented into individual pages. Each of the images on a first side a sheet must therefore be registered with a corresponding image on a second side of the sheet before the sheet may be segmented into individual pages.
Specifically, the first image that appears on the first side of the sheet and the second image that appears on the second side of the sheet are positioned so that identical images printed on both sides of the sheet are coincident with each other. In other words, two identical images printed on both sides of a sheet of media sheet form mirror images of each other since each image is printed with no apparent offset from the other. Thus, an image on the front side of a sheet would appear to be in perfect or transparent registration with the corresponding image on the back side of the sheet.
The “show through” error that occurs when transparent registration is not achieved can be quantified by measuring of the displacement between two points, one on a first side of the sheet and one on a second side of the sheet, that are intended to be equidistant from a common sheet edge. This error is caused, at least in part, by the factors identified above. The portion of the error associated with media sheet shrinkage is often caused by fusing a printed image on the first side prior to printing of an image on the second side.
Accordingly, it is desirable to provide a sheet registration method and system for precisely registering an image on one side of a sheet with an image on the reverse side.